Add SwiftPM Support

[jurvis]

cherry picked from #69. wip.

Test Plan

• Upload SwiftPM release to Sjors/libwally-swift
• Update Package.swift to point to Sjors/libwally-swift

[Sjors]

So does this checksum allow others to verify that binary is generated from the code?

[jurvis]

unfortunately not, the xcframework builds are non-deterministic, so they do change between builds. I'm currently working on a bunch of CI actions so we won't have to do releases manually + users won't have to trust us.

we did something over at https://github.com/lightningdevkit/ldk-swift/actions/runs/2819921873 where we use CI to build releases that...

1. Generates the XCFramework
2. Calculates the checksum of the framework from step 1 and use it to modify lines 4 and 5 here
3. Publishes the release

Though, of course, it does mean that users will have to trust GitHub. But I think it is Good Enough™ to get a SwiftPM option to work nicely, while keeping Cocoapods around for developers who prefer a more "do it yourself" option.

What are your thoughts?

[Sjors]

I think it's fine, as long as we make it clear in the documentation (README?) that using SwiftPM means trusting Github.